Batch thermal processing, in which multiple wafers are simultaneously processed in a furnace, continues to be widely practiced in the semiconductor industry. Most modern batch thermal processing is based on vertical furnaces in which a vertically arranged support tower holds a large number of wafers in a horizontal orientation. The towers are conventionally composed of quartz, especially for processing temperatures under 1000° C. or of silicon carbide, especially for higher processing temperatures, but silicon towers are entering service in commercial use for all temperature ranges.
One process that utilizes such thermal processing is high temperature oxidation (HTO), in which very thin oxide layers are grown by chemical vapor deposition (CVD) using SiH4 and N2O or NO as precursor gases. Typical CVD temperatures are in the neighborhood of 750° C. The thin oxide may have a thickness in the vicinity of 2.5 nm or less and be used for a tunneling barrier, for example, in flash memories. Other processes are available for growing thin films, such as using O2 as an oxidizing agent.
Thickness uniformity of the grown film has, however, been a problem. A thickness profile 12A is schematically illustrated in the graph of FIG. 1. Two peaks 14A, 16A in the thickness have been observed near the wafer periphery. The peaks 14A, 16A may represent 16% and 33% variation on opposed sides, and, since tunnel current varies exponentially with thickness, a modest thickness variation can produce a large variation in tunneling current and hence the recording performance of flash memories.
The specific origin of the peaks is not completely understood, but possible causes are believed to include thermal edge effects such as thermal shadowing by the tower legs or proximity to the furnace wall, and by gas flow discontinuities at the wafer periphery. Some have attempted to solve this problem by attaching auxiliary rings to the tower which extend over the edge of the wafer a small distance toward the center. Optimally, the wafer is spaced between the two neighboring edge rings facing its upper and lower faces. Edge rings have been shown to be effective at reducing if not eliminating the peaks.
The typical design includes a quartz tower and quartz edge rings which are fused with the three or four legs of the tower. This design suffers several problems. Although the quartz is relatively inexpensive, the fusing at so many locations is laborious. If one of the edge rings is broken in service, repair is almost impossible. Either the tower and welded edge rings are discarded or the wafer locations around the broken edge ring are not thereafter used for production wafers. Although quartz is generally accepted for use in thermal support fixtures, advancing technology calls into question whether it has an adequate purity level.
Accordingly, a better design is desired for edge rings and their support towers.